The present invention relates generally to optical Kerr gate systems and more particularly to a multiple-stage optical Kerr gate system.
Optical Kerr gate systems have become well-known over the past twenty years having been used primarily as simple ultrafast shutters to measure ultrafast events and relaxation processes in applications as varied as imaging communications and digital computation. Generally speaking an optical Kerr gate system comprises a light source for producing an intense gating pulse and an optical Kerr gate comprising a non-linear medium (i.e. a Kerr cell) disposed between a pair of crossed polarizers (the second polarizer often referred to as an analyzer) In the absence of a gating pulse sent contemporaneously through the Kerr cell, a signal pulse of arbitrary polarization entering the Kerr gate along its longitudinal axis will not emerge therefrom. This is because the signal pulse will be polarized in a first direction by the first polarizer, will go unchanged through the Kerr cell, and, will be blocked from passage by the analyzer. In contrast, if the gating pulse is directed onto the non-linear medium while the signal pulse is also incident thereonto, a transient birefringence is introduced into the Kerr cell causing the signal pulse to become elliptically polarized and permitting at least a portion thereof to pass through the analyzer.
For a typical optical Kerr gate using a pair of Polaroid sheet polarizers, gated pulses having a signal to noise ratio (S/N) of approximately 2.times.10.sup.3 and a time duration of approximately 8-16 picoseconds (ps) can frequently be achieved using a 10 ps gating pulse from a mode-locked glass laser. Using a 100 femtosecond (fs) gating pulse from a dye laser and either lead glass or a polymer as the Kerr medium, 100 fs gates can be achieved. Unfortunately in numerous applications, such as in the time-gated optical imaging of various biological and medical specimens, gated pulses having higher S/N ratios (frequently as high as or greater than 10.sup.10) and shorter durations than noted above are often desirable.